FIG. 1 and FIG. 2 are top and cross-sectional views respectively of a prior art, dual, differential pressure sensor 1. The pressure sensor 1 is referred to as a dual differential pressure sensor because it includes two separate differential pressure sensing transducer dies. Each transducer die is a differential pressure sensor.
As used herein, the term differential pressure sensor refers to a pressure sensor that measures or senses an amount by which the pressure existing in a liquid or gas differs from a reference pressure, typically atmospheric pressure. The device shown in FIG. 1 and FIG. 2 is therefore able to measure two differential pressures, e.g., the pressure on two different liquids or the pressure drop at two differential pressures along a flow path and, provide two corresponding output signals that are representative of the differences between the two measured pressures and a reference pressure.
The pressure sensor 1 is comprised of a housing 2 with a pocket 3 formed by or defined by a vertical side wall 4 that wraps around the pocket 3. The pocket 3 has an open top and a planar or substantially planar bottom surface 5. Two small ports 6, which are visible in the cross-sectional view of FIG. 2, extend from an exterior bottom surface 7 of the housing 2, “upwardly” as shown in FIG. 2 and into the pocket 3 through the planar bottom surface 7. Two separate piezoresistive transducer (PRT) dies 8, which are well known in the sensor art, are mounted on separate, corresponding pedestals 9. The pedestals 9 rest in a bead of adhesive 50, which surrounds each port 6.
The adhesive 50 has an uncured liquid state and a substantially solid cured state. Uncured adhesive is applied to the top of the planar bottom surface 5 as a bead of liquid. The adhesive 50 is applied so that it completely surrounds both ports 6. The pedestals 9 with the PRT dies pre-attached to them are placed on top of the bead of adhesive 50 before the adhesive 50 cures. The pedestals 9 thus settle into the uncured adhesive 50. When cured, the adhesive 50 mechanically bonds the pedestals 9 to the planar bottom surface 5. It also provides a seal between the planar top surface 5 and the pedestal 9.
A viscous gel 75 partially fills the pocket, substantially as shown. The gel 75 protects the PRT dies but it also allows ambient pressure to be applied to the upward facing side of the PRTs, i.e., the side of the PRTs facing ambient pressure.
The gel 75 is viscous but flexible enough to allow ambient air pressure to exert a downward force on the top side of the PRT dies 8. The aforementioned ports 6 carry a media upwardly as shown, the pressure of which exerts an upward force on the PRT dies. The PRT dies are thus subjected to two forces: ambient pressure and the pressure inside the corresponding ports 6. The PRTs measure the difference between those two pressures and are thus referred to as differential pressure sensors.
A problem with the prior art pressure sensor shown in FIGS. 1 and 2 is that the cross-sectional area of the ports 6 is small. If too much of the viscous but nevertheless liquid adhesive 50 is applied around the ports 6, it can roll into one or both ports 60, partially or even completely blocking the port or ports 6. Once the adhesive 50 cures inside a port 6, the adhesive 50 cannot be easily removed from the port 6, without damaging the device 1. A second and related problem is that while many users of the device 1 prefer the overall dimensions to be reduced, reducing the overall size of the device tends to increase the likelihood that adhesive will fall into the ports 6. Reducing the size of the device 1 and keeping the ports 6 clear of adhesive 50 during assembly is therefore problematic.
The problem of adhesive overflow into the ports could be solved by increasing the space or distance between the ports 6 and the adhesive 50 but increasing the space between the ports 6 and the adhesive 50 would require the overall size of the device 1 to be increased. Unfortunately, the trend in all fields of electronics, including pressure sensing, is to make devices smaller rather than larger. A pressure sensor having two or more separate PRTs and which has a reduced size but which also can avoid or eliminate adhesive 50 blocking the pressure ports 6 would be an improvement over the prior art. Another manufacturing process advantage is that a single PRT die 60 with two PRTs can reduce eight bond pads to six bond pads 95, which also reduces the number of connecting wires to the die from eight to six.